In an internal combustion engine, it is known to deliver fuel into the cylinders of the engine by means of a fuel injector. One such type of fuel injector that permits precise metering of fuel delivery is a so-called ‘piezoelectric injector’. Typically, a piezoelectric injector includes a piezoelectric actuator that is operable to control an injection nozzle. The injection nozzle houses an injector valve needle which is movable relative to a valve needle seating under the control of the actuator. A hydraulic amplifier is situated between the actuator and the needle such that axial movement of the actuator causes an amplified axial movement of the needle. Depending on the amount of charge applied to, or removed from, the piezoelectric actuator, the valve needle is either caused to disengage the valve seat, in which case fuel is delivered into the associated engine cylinder through outlets provided in a tip of the nozzle, or is caused to engage the valve seat, in which case fuel delivery through the outlets is prevented. The amount of charge is varied causing the valve needle to move between closed and open positions.
The amount of charge applied to and removed from the piezoelectric actuator can be controlled in one of two ways. In a charge control method, a current is driven into or out of the piezoelectric actuator for a period of time so as to remove or add, respectively, a demanded charge to or from the stack, respectively. Alternatively, in a voltage control method a current is driven into or out of the piezoelectric actuator until the voltage across the piezoelectric actuator reaches a demanded level. In either case, the voltage across the piezoelectric actuator changes as the level of charge on the piezoelectric actuator varies, and vice versa.
In order to initiate an injection of fuel, the drive circuit causes the differential voltage across the actuator terminals to transition from a high level at which no fuel delivery occurs to a relatively low level to initiate fuel delivery. An injector responsive to this drive waveform is referred to as a ‘de-energise to inject’ injector. When in a non-injecting state, in which the actuator spends most of its life, the voltage across the de-energise-to-inject injector is therefore relatively high and when in an injecting state the voltage across the actuator is relatively low.
It has now been recognised that the existence of such a high voltage across the actuator terminals for a relatively long portion of the injection cycle may adversely affect the injector. This is thought to be attributable, in part, to the fact that the higher the voltage across the injector, the higher the stress the actuator is subjected to when in a non-injecting state. It is also suspected that a high voltage across the terminals may encourage the permeation of ionic species into the actuator though its protective actuator encapsulation. In any event, inaccuracies in fuel volume delivery have a detrimental effect on combustion efficiency and lead to worse fuel economy and increased exhaust emissions.
It is an object of the invention to provide a method of operating a piezoelectric fuel injector so as to reduce or alleviate the aforementioned disadvantages.